After the harvest the harvested material is usually stored in large amounts in storage rooms until the further processing or sale. During this storage so-called storage diseases can develop which may damage the harvested material. Storage diseases do not only result in direct deficiencies of the harvested material, but may also cause a permanent deterioration of quality due to a damage of the reproduction cascade of the plant material.
Well-known storage diseases of the potato plant (Solanum tuberosum) are tuber blights and scurf diseases. The brown blight, the potato pox, the wet blight and the potato dry blight belong to the tuber blights. The powdery scurf and the silver scurf belong to the scurf diseases.
When it comes to the quality of potatoes, the silver scurf comes more and more to the fore. It becomes noticeable through its silvery grey-brown spots formed on the skin of a potato. The pathogenic agent of the silver scurf is the fungus Helminthosporium solani. It is unrelated to the ordinary potato scurf which is induced by a bacterium, or to the powdery scab which is induced by a lower fungus. Helminthosporium solani belongs to the class of the ascomycetes, the so-called real sac fungi. Helminthosporium solani only affects the potato tuber. It cannot be found at the potato plant above the ground. The disease is limited to the surface of the tuber. At the time of the harvest slightly recessed, brownish spots are visible. In the course of aging the spots become darker and appear silvery. The affection can spread over the entire surface of the tuber. The conidia carriers of the fungus with the dark spores are predominantly located on the brink of the spots and may confer them a fuliginous appearance. The silvery spots result from the pathogenic agent developing in the cork layer of the tuber skin and detaching it from the cortical parenchyma. The destruction of the tuber skin favors the natural loss of water through evaporation since the skin is abrogated as a natural evaporation barrier. The affection by silver scurf therefore results in a weight reduction of potato tubers and a premature dehydration. Another damage is based on the fact that the so-called eyes or germs of the potatoes may be affected and killed resulting in a reduced and incomplete sprouting of affected harvested material. As a result the potato tuber is no longer edible.
The development of storage diseases such as the silver scurf is currently essentially prevented by the heating of the harvested material. Various chemicals are available for this purpose. The most important chemical is the Imazalil which is e.g. distributed under the trade name Magnate®. A drawback is that Imazalil is hazardous to the environment and harmful to the health. Imazalil is toxic for fish and causes damages to birds and ducks. The half-life for the degradation in the ground is about half a year. Imazalil remains in the ground and is not washed out.